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The Latest technological development in the manufacture of automotive alloy wheels: cast flow-forming

Aluminum wheel manufacturing has evolved
from the garages of entrepreneurs in the early 1950's in Southern California to
a global large scale and high volume industry.

Wheels have traditionally been cast
in permanent molds, initially by manually poring (gravity) the molten alloy (Aluminum
Alloy A356) through the center of the mold, and subsequently, and currently the
industry standard, using sophisticated low pressure machines, whereby the
molten metal is injected from the bottom into the permanent mold. Solidification
of the molten alloy is realized by cooling, with water and/or air. A
solidification cycle could vary form 4-12 minutes depending on the size of the
wheel cast and cooling techniques.

Gravity and Low Pressure Casting
have been the standard cast technologies worldwide for the last 40 years, many
low-cost producing nations now use low pressure machines like many of their
counterparts in North America and Europe. Both of these techniques have been perfected
and can yield up to a 95% quality rate (castings free of inclusions, mayor pores,
voids, cracks). 

Over the last 20 years, a new
technology for manufacturing automotive wheels was introduced in the USA and
Europe called forging of wheels. Forging of wheels is done with mechanical
presses of large tonnage, starting from an alloy log formed into a donut and subsequently
spun into a wheel shape.

The alloy used to forge wheels is
generally a 6000 series aluminum alloy; some manufactures use 5000 series as
well. These alloys are standard in the aerospace industry. Forged wheels are
lighter than cast alloy wheels by as much as 15%-25% because of thinner rim
walls and in some areas of the wheel better mechanical properties are achieved.

The weight reduction of forged
wheels is appealing to the automotive manufacturer because it reduces the “un-sprung
weight” of a vehicle thereby improving fuel efficiency. For the consumer, they
are appealing because they are perceived to be more attractive and expensive to
manufacture, which in most cases they are, and use the “aerospace alloy”. A disadvantage
of forged wheels is that wheel faces are not as attractive as cast wheel faces
because shapes and radii are very difficult to manufacture via the forged
process, unless a five (5) axis milling process is used in the machining
process, which makes the forged wheels even more costly to produce.

Approximately ten years ago a new
process was developed that used the main advantages of cast and forged wheels
and was named flow-forming of alloy wheels. This relatively new process
produces wheels with the aesthetic advantages of cast faces and the weight
savings of forged rim sections, thereby marrying both manufacturing processes.

Flow-forming of wheels are first
cast in a permanent mold, preferably in a low pressure machine, but instead of
a complete cast rim section, a three to four inch rim is cast, slightly thicker,
compared to standard wheels. Subsequently, the wheels are pre-heated and loaded
in a flow-forming machine of two or three rollers that will extend (flow-form)
the 3”-5” rim section into a full-size rim section of 7”-8” length and fully
contoured for the tire. This flow-formed section will be as thin as a traditional
forged rim section. The flow formed wheel will have a cast-face and a thin rim,
comparable to forged wheel, but with the aesthetics of a cast face, best of
both worlds!

Please refer to the flow-forming
video from Industrias Puijganer of Barcelona, Spain for a demonstration of flow-forming
of a wheel:


This process is currently being
used to manufacture large size Original Equipment wheels (20”-22” diameters)
for large SUV’s and luxury vehicles and the automotive manufacturers are paying
a premium for these wheels. A Korean automotive manufacturer is now using this
process to make smaller diameter wheels to improve fuel efficiency of smaller




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